This is an extrafloral nectary:

Extrafloral nectaries, or “EFNs”, are little sources of sugar and nitrogen produced to entice ants to visit host plants. The plants, in return, enjoy the significant defensive capabilities of the ants, which often repel herbivorous insects. Given this benefit, it is perhaps not surprising that many different plant species and clades have evolved EFNs. But although these conspicuous structures have received much attention in the scientific literature, ants also benefit from a different, understudied “structure”: plant wounds.

Like EFNs, plant wounds (typically caused by herbivore munching) also excrete sugar and amino acids. Of course, this only further damages the plant – or, does it? The authors of a recent study in The American Naturalist investigated the “ant-wound network” in subtropical Jianxi, China, and also considered the implications of their findings for theories of EFN evolution.

The researchers (Dr. Michael Staab and colleagues) examined leaves from 10,000’s of trees over several years, documenting cases of ants feeding at herbivore-induced plant wounds and analyzing the nutrients present in wound sap via high-performance liquid chromatography. Their key finding was that 22 species fed on plant wound sap and did not exhibit specialization – plant wound feeding appears to be a facultative, opportunistic behavior in ants. Furthermore, the ant community feeding at plant wounds were a subset of the community that tend Hemipteran (“true bug“) mutualists.

Besides being a very nice documentation of an under-appreciated and apparently widespread ecological interaction, what do these results imply? In an enjoyable Discussion section, Staab and colleagues offer some suggestions. One particularly interesting hypothesis is that plant wounds are an evolutionary bridge to EFNs, an idea that is supported by the widespread and functionally similar nature of plant wounds uncovered in the study. The researchers also posit that ants feeding on plant wounds may not actually be mutualistic, and the food source may simply provide the ants some extra energy on their way to more nutritious food sources. If this is the case, the remarkable ecological dominance of ants in forest canopies may partially be supported by wounded plants – a little morbid, but very intriguing!